Microsoft Xbox Live VP Marc Whitten has told The Verge that the Xbox One will not have backwards compatibility. This is due the console having a different core architecture than the Xbox 360.
"No, there's not," said Whitten when asked about backwards compatibility. "The system is based on a different core architecture, so back-compat doesn't really work from that perspective."
The Xbox One uses an x86 CPU, while the Xbox 360 uses a Xenon processor. Games won't be able to run natively on the new hardware due the difference. The PlayStation 4 has the same problem, but Sony did promise PlayStation 3 games will be playable at some point through the cloud.
It is possible for the Xbox One to do the same thing in the future, but nothing has been announced at this time.